U.S. Pat. No. 4,919,153 teaches a cement extraction system wherein the old cement mantle left after removal of a prosthetic appliance is extracted by injecting new cement into the cavity left in the mantle by removal of the appliance and then engaging a threaded pulling tool with the new cement. In that system, the entire mantle, with the new cement therein, is removed as a unit. The system of U.S. patent application Ser. No. 475,778 is concerned with a variation wherein a die is used to form a screwthreaded passage in the new cement and then pulling tools are successively engaged with segments of the passage to incrementally remove the mantle. This "incremental removal system" has the advantage that the mantle and the bone within which it is received is subjected to reduced stress, as compared to the system of U.S. Pat. No. 4,919,153.
The system of the Ser. No. 475,778 application uses separate extraction rods with a slap hammer to incrementally remove cement segments typically about one inch in length. If the old mantle is very tightly adhered to the bone inside the femoral canal, the force required to extract a segment may exceed the strength of the cement threads. Upon slap hammer use, the extraction rod may eventually pull out of the new cement without an attached cement segment.
Another cause for cement thread strippage with the system of the Ser. No. 475,778 application is the possible malpositioning of the thread forming rod during its placement in the new cement. If the rod is not centered within the cavity in the old mantle, but instead lies against the side of the mantle, insufficient new cement lies between the thread forming rod and the old cement mantle on that side. As a result, incomplete threads may be formed on that side, leading to strippage of the extraction rod from the cement segment upon slap hammer application.